Wave division multiplexing (WDM) optical networks are well known. A WDM channel is typically transmitted by a single mode semiconductor laser, where information to be communicated is imposed on the light by modulating the laser current or by externally modulating the light by applying a voltage to a modulator coupled to the laser source. A receiver subsequently photo-detects and converts the light into electric current either by direct or coherent detection.
Due to the rapid growth of optical networks and the need for greater capacity, significant research has focused on finding efficient multi-level optical modulation formats. Any digital modulation scheme uses a finite number of distinct signals to represent digital data. Phase-shift-keying (PSK) uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits, and each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thereby recovering the original data. The receiver compares the phase of the received signal to a reference signal. This expedient utilizes coherent detection and is referred to as CPSK.
Alternatively, in lieu of using the bit patterns to establish the phase of the wave, CPSK employs differential phase changes. The demodulator then determines these phase changes in lieu of the actual phase of the signal. This scheme is referred to as differential phase-shift keying (DPSK), and is easier to implement than PSK as there is no need for the demodulator to maintain the reference signal to determine the exact phase of the received signal.
BPSK (also sometimes called PRK, Phase Reversal Keying) is the simplest form of PSK. It utilizes a pair of phases separated by 180° and is known as 2-PSK.
Quaternary or quadriphase PSK, 4-PSK, or 4-QAM (QPSK) uses four points on a constellation diagram as is known in the art. The four-phase QPSK can encode two bits per symbol—twice the rate of BPSK—and experimentation has demonstrated that this may double the data rate compared to a BPSK system while maintaining the bandwidth of the signal. Alternatively, QPSK can maintain the data-rate of BPSK at half the requisite bandwidth.
Optical modulations based on four-level quadrature-phase-shift-key (QPSK) have been effectively demonstrated for both 40 Gb/s and 100 Gb/s optical transmission. In the quest for even higher spectral efficiency, eight-level 8-PSK modulation has been proposed and demonstrated experimentally.
8-QAM is another eight-level modulation format. In comparison to 8-PSK, 8-QAM is tolerant of greater noise (on the order of 1.6 dB), with identical spectral utilization.
FIG. 1 is a schematic of an 8-PSK modulator 100, which comprises an optical splitter 104 that splits the incoming light from a CW laser source 102 into two components—a first part 106 and a second part 108. The first and second parts 106, 108 are modulated by Mach-Zehnder Modulators MZM1 110 and MZM2 112, which are driven by binary signals DATA1 and DATA2, respectively, and biased at the null point with a driving swing of 2 Vπ. The modulated lower part from MZM2 112 is applied to a phase shifter 114 to impose a phase shift of π/2. The modulated first part 106 and modulated and phase-shifted lower part are combined by a 1:1 optical combiner 118 and the output thereof subsequently phase-modulated by (0, π/4) with binary signal DATA3 at phase-modulator 120 to produce the 8-PSK signal.
8-QAM encodes the signal in both amplitude and phase, thus making 8-QAM more difficult to practically implement than 8-PSK.